Temperature and humidity control means for refrigerating system



May 17, 1966 J. E. WATKINS TEMPERATURE AND HUMIDITY CONTROL MEANS FOR REFRIGERATING SYSTEM 2 Sheets-Sheet 1 Original Filed Dec. 6, 1961 5.502% E. wm m E N om INVENTOR. JOHN E. WATKINS #MW;

ATTYS.

BY J6 May 17, 1966 Original Filed Dec. 6, 1961 J. E. WATKINS 3,251,196

TEMPERATURE AND HUMIDITY CONTROL MEANS FOR REFRIGERATING SYSTEM 2 Sheets-Sheet TEMP E JOHN E. WATKINS ATTYS.

United States Patent 3,251,196 TEMPERATURE AND HUMIDITY CONTROL MEANS FOR REFRIGERATING SYSTEM John E. Watkins, Maywood, 111., assignor to Central Refrigeration Systems, Inc., Medford, Mass., a corporation of Massachusetts Continuation of application Ser. No. 157,351, Dec. 6,

1961. This application May 6, 1964, Ser. No. 366,222 9 Claims. (Cl.- 62-158) This is a continuation of application Serial No. 157,351, filed December 6, 1961, and now abandoned.

This invention relates to refrigeration and more particularly to improvements in controls for refrigerating systems.

The principal aim of the invention is to increase the efficiency of operation of refrigerating systems, especially, although not exclusively, refrigerating systems for cooling storage rooms for shrinkable products, such as meat. I

An important object of the invention is to provide improved means for controlling temperature and humidity in such refrigerating systems wherein such control is achieved with the refrigerating unit operating at full capacity without throttling of the flow of liquid refrigerant through the evaporator coil.

Another object of the invention is to provide an improved temperature control means for refrigerating systems which control means will be effective to hold temperatures precisely within very small differentials. Another object of the invention is to provide an extremely sensitive temperature responsive device for such temperature control means, which device is free from chattering which normally constitutes a disadvantage in the use of a temperature-responsive device having high sensitivity in such application.

Other objects and advantages will become apparent as the following description proceeds, taken in connection with the accompanying drawings, in which:

FIGURE 1 is a diagrammatic illustration of a refrigerating system and temperature and humidity control means therefor;

FIG. 2 is a view in elevation with fragments shown in section of a thermostat for. the temperature control unit shown in FIGURE 1; and

FIG. 3 is a chart depicting curves which will be referred to in explaining the invention.

While the invention has been shown and will be described in some detail with reference to a particular embodiment, there is no intention that it be limited to such detail. On the contrary, it is intended here to cover all modifications, alternatives and equivalents falling within the spirit and scope of the invention as defined in the appended claims.

Refrigerating system organization As an example of how the invention is applied in practice, reference is made to FIGURES 1 and 2 which show an evaporator cooling coil supplied through a liquid line 12 with liquid refrigerant from a source including the expansion valve 13, solenoid valve V, and receiver 14. The suction line 16 is connected to the compressor 18 which, in turn, is connected to a condenser 20 and the condenser to the receiver 14 in the usual manner.

The refrigerating system comprising the evaporator 10 and the source of liquid refrigerant, is applied in the present case to the refrigeration of a storage space, illustratively, a storage room for beef in a meat packing plant. Certain conditions of temperature and humidity are re quired to be maintained in a room where beef, which is a shrinkable product, is chilled and stored. Beef chilling is done preferably at a temperature of about F. since beef freezes at about 28 and at higher temperatures the circulate air over the coils.

chilling process is too slow. It is desirable, therefore, to maintain the air at such temperature in the storage room and, further, at a relative humidity of about 92% since at that point shrink due to moisture pickup from the product is low. It will be understood that the refrigerating system in connection with which the invention will be explained is such as to hold a 30 temperature and 92% relative humidity (RH) in the storage room. The capacity of the system in terms of total tons of refrigeration may be as large or as small as needed and in keeping wit-h this and for convenience in explanation, the description of the apparatus will be in per ton terms. A ton of refrigeration, for definition purposes, is equivalent to the transfer of 200 Btu. per minute.

Referring again to FIGURES l and 2, in addition to the evaporator 10 and source of liquid refrigerant for such evaporator, the refrigerating system includes a number of fans 21-23 for circulating air over the coils of the evaporator and between the latter and the storage room.

Heretofore, to control relative humidity and temperature in such systems, the usual practice followed has been to vary the evaporating temperature in the heat absorber, for example, by modulating the flow of refrigerant through the absorber. By this means, moisture dropout from the air circulated between the storage room and the evaporator is controlled so that the relative humidity is maintained at the desired point for humidity control. Likewise, temperature control is obtained according to the rise and fall of heat absorber temperature.

Such systems, where temperature or temperature and humidity are regulated by varying the heat absorber temperature with constant volume of air flow past the heat absorber, suffer a loss of efiiciency whenever the flow of liquid refrigerant through the heat absorber is throttled to increase temperatures. In my copendin-g application, Serial No. 142,977, now abandoned, entitled, Controls for Refrigerating Systems, I have disclosed means for regulating humidity in such systems by operating the evaporator coils at substantially full capacity, thereby obtaining higher efficiency of operation, and by controlling the volume of air circulated over the coils such that the temperature of this air is varied according to the rate of moisture removal required to maintain the desired humidity point.

Humidity control To maintain air in the storage room at 30 F. and 92% relative humidity, the condition specified for the meat storage room of our example, requires extremely close control over both humidity and temperature. As any standard psychrometric chart shows, air at 30 F. and 92% relative humidity contains moisture or water vapor in a superheated condition. By cooling this air the degree of superheated vapor decreases, finally reaching a point, called the dew point, at which the water vapor becomes saturated. Any further extraction of heat will cause condensation. According to the psychrometric chart, air at 30 and 92% relative humidity reaches the dew point by being cooled through a range of 19 F.

In my copending application referred to, the humidity control system disclosed operates by controlling the volume of air circulated past the evaporator coils, thereby to vary the moisture removal from the air. According to the invention disclosed in that application, in the normal operation of the system where the humidity is at the specified condition, all three fans 21-23 operate to At the volume of circulation provided by such three fans, the air passing over the cooling coils is cooled less than the range of 1.9 below the specified storage room temperature of 30 F. in order to maintain the cooled air above the dew point. Under such conditions there is insubstantial moisture fallout in the form of frost on the coils and the relative humidity,

will be held at the specified level or will rise due to marginal condensation on the coils being picked up by the circulated air and carried to the storage room. Upon rise in relative humidity above the specified point, the humidity control responds to shut ofi": one fan 22 and thereby reduces the volume of air circulated so that the air passing over the cooling coils is cooled more than the range of 19 and to a temperature, consequently, below the dew point. Under such conditions where the air is cooled below the dew point, there is substantial frost forming moisture fallout, reducing the relative humidity of the air in the storage space.

It will be observed that the temperature within the storage space to obtain proper operation of the humidity control, must be controlled highly accurately. Unless the temperature is held accurately to about the specified temperature of 30 B, it will become difficult if not impossible to hold the temperature at the evaporator coil within the 1.9 F. range so as to hold the temperature of the air passing over the coils off the moisture dropout curve, that is, above the dew point so as to prevent moisture fallout or condensation to afford the humidity control provided by the system.

To this end, referring to FIGURE 1, regulating the temperature of the storage room is achieved by interrupt ing the fiow of refrigerant through the evaporator coil by means of a liquid solenoid valve V responsive to the temperature in the space to be cooled. The coil operates flooded without throttling of the liquid refrigerant. temperature responsive unit 30 constructed in keeping with the invention, is connected in the power circuit to the liquid solenoid valve -V so that when the temperature rises above the set level in the storage room, the valve V is opened to supply liquid refrigerant to the coils. When storage room temperature drops below the setting of the temperature responsive unit, the solenoid valve V is closed. The fans 2123 which serve as an air circulating means will move a certain volume of air past the evaporator coils which function as a heat absorber to extract heat from the circulating air. 'It will be understood that the temperature drop of the air in passing over the coils determines the cooling effect in B.t.u./ hour or equivalent tons of refrigeration which will be produced by the refrigeration unit. The fans are kept in steady operation, as above noted, except when one is stopped for humidity control purposes or for coil defrosting.

The humidity control is arranged so that responsive to the humidistat calling for an increase in relative humidity, a high volume of air is circulated past the evaporator coils, so that the cooled air is maintained above the dew point thereof. This has the effect of substantially eliminating any moisture fallout from the air by condensation on the evaporator coils, such that relative humidity in the storage room is caused to be maintained or to rise due to evaporation of previously condensed moisture or moisture pickup off the product. If such air-temperature conditions at the coils are maintained without change, the relative humidity in the storage room rises unimpeded until it reaches the dew point. This condition is to be avoided in the present embodiment of the invention, since experience shows that if relative humidity substantially higher than 92% is maintained in storage rooms for shrinkable products, such high relative humidities will cause sliming and souring of the shrinkable products, such as meat. For this reason, it is desirable to hold the relative humidity at the prescribed point of 92%, although it will be understood that the particular point can be varied as desired and that 92% is chosen as an example only.

By regulating the volume of air passing over the coils responsive to relative humidity, increasing the air volume circulation when relative humidity drops so as to raise the temperature of the air passing over the evaporator coils toabove the dew point, and decreasing the air volume circulation when relative humidity rises so as to drop the temperature of the air passing over the coils to lower than the dew point, the objective is obtained of maintaining a predetermined desired relative humidity in the storage room.

For regulating the volume of the air circulated responsive to relative humidity, a humidistat 31 is connected in the AC. line to the center fan 22. It is contemplated that the humidistat 31 will be set to break its contacts upon rise in relative humidity above the set point thus causing the center fan to stop running and decreasing the volume of air being circulated. This in volume of circulation results in change in temperature at the evaporator coils. With the three fans running, the air volume circulated is such as to maintain the temperature of the cooled air above the dew point. At the lower volume of air with only two fans running and the center fan otf, the temperature of the circulated air is lowered below the dew point. As pointed out in the copending application, While air circulating regulating means is shown as separate fans, other means might be used.

Brief mention will be made of the bypass thermostat 32 which is also shown in FIGURE 1. This serves as ashort circuiting means with its contacts in parallel circuit with the humidistat contacts. The bypass thermostat 32 will 'be set to close it contacts at a temperature 4 to 5 higher than the setting of the room thermostat so as to override the humidity control for any period of abnormally high temperature. Such abnormally high temperatures are encountered when a storage room is freshly filled with warm beef. In such case, the meat product will sweat, producing a temporary high humidity and high temperature condition. The bypass thermostat 32 in this arrangement will be responsive to. the abnormal rise in temperature indicative of the high temperature-high humidity condition to connect all three fans 21-23 across the AC. line so that the unit will function in full cooling capacity to cool the room down to within the specified temperature range. As soon as the temperature is within the specified range, the humidity control comes into operation.

Temperature control The relationship between the temperature control and humidity control will be better understood by considering that the relative humidity control depends upon controlling air flow so as to provide predetermined, highly critical temperature conditions at the evaporator coil. It will be clear, as pointed out above, that the temperature of the air in the storage room should be held within close limits. To have proper operation of the humidity control, the lower limit of the temperature at which the air in the storage room is maintained must be held by the temperature control substantially above the' dew point in order to allow the requisite range of cooling at the evaporator coil without moisture dropout at the high volume of circulation. Otherwise, moisture fallout will occur under either condition of air circulation volume interfering with the proper operation of the humidity control system.

Before turning to the apparatus aspects of the temperature control unit, the chart of FIG. 3 is referred to for illustrating the results obtained using the unit. The large amplitude saw-toothed curve A shown in this chart is a plot of the temperature variations with a refrigeration system provided with the usual temperature control. Thus temperature is shown in F. along the ordinate at the left, while time is shown along the abscissa at the top of the chart in 30-minute increments. It will be seen that the amplitude of the curve shows a total temperature rise and fall of 4 F. This 4 change in temperature is achieved in 30-minute periods in the example, wher the slope of the curve is the same for both rise and fall in temperature.

This same refrigeration system provided with a small differential temperature control unit constructed according to the invention, will have temperature fluctuations which follow the small (B) amplitude saw-toothed curve appearing at the left side of the chart between the limits of approximately 30.1 F. and 29.9 F. This small amplitude curve is plotted using the same time scale at the top of the chart. In other words, with the temperature control unit of the present invention, the storage room temperature is held within a range of about .2 F. The rate of change for both rise and fall in temperature is the same. The slope is the same as the slope of the straight lines making up the large saw-toothed curve.

In order to show the details of the small amplitude saw-toothed curve depicting temperature change, the time base has been expanded and is marked along the abscissa at the bottom of the chart, and the same saw-toothed curve has been plotted using the expanded time base and for this purpose is superposed on the other curve, as curve C.

Referring now to FIG. 1, the temperature control unit 30 of this invention includes a thermostat 33 in a control circuit in series with the energizing coil 34 of a time delay relay 36. The thermostat contacts 38a, 38b of the control circuit are arranged to make and break responsive to temperature to energize the time delay relay 36. The time delay relay contacts 49 are in the energizng circuit for the liquid solenoid valve V in the liquid line to the evaporator coils.

A suitable thermostat for this unit is shown in FIG. 2, and is a Fenwal differential expansion thermostat known as Series 17,000. This thermostat includes an external metal shell 42 which exands or contracts with temperature changes. Inside the shell are mounted contact points 38a, 38b of silver or like high-conductance ma teral which are supported by non-expanding struts 44. In the construction shown, each contact point 38a, and 33b is connected to a conductor 46a, 4612 which leads outside the casing. The conductors 46a, 46b are connected, as shown in FIGURE 1, in the control circuit in series with a source of current and the coil 34 for the time delay relay 36 of the unit 30. In the type of thermostat utilized in the temperature control unit 30, the contacts 38a, 3852 are supported and arranged to close or make on rise in temperature, that is to say, the contacts make on rise in temperature to the set point of the thermostat 33. This set point may be adjusted conveniently and readily by means of the adjusting sleeve 48. The adjusting sleeve 48 is threaded into the end cap 50 of the shell 42 and is internally threaded to receive a threaded stud 52 that is fixed to the contact strut assembly. The outer threads 54 on the sleeve 48 are coarser than the internal threads 56 such that upon turning the sleeve 48, the stud 52 is caused to move endwise and via the struts 44 to adjust the spacing of the contact points 38a, 33b. The contact strut assembly is fixed by a pin 58 to the opposite end cap 60 of the shell 42,

Certain of the characteristics of the temperature responsive element are of particular significance. It has a fast reaction, of the order of about seconds. It is responsive to an extremely small temperature differential on the order of .l F. under temperature change condi tions in the type of installation dealt with here. In addition, this type of thermostat has a low cost of manufacture as compared with more complicated correspondingly sensiture bi-metallic or bulb-type thermostats. With such a relatively light and inexpensive electrical construction its contacts 38a, 38b are not built to provide many cycles of operation under heavy load currents, and under such load conditions contact wear will become excessive.

This type of thermostat with its high sensitivity is completely adequate for use in the temperature control unit of the present invention where the thermostats contacts 38a, 38b carry control currents of only a few milliamperes' while the relay contacts 40 carry heavier load currents, and such thermostat will provide many cycles of operation under these conditions. Accordingly, a temperature control unit 30 is provided which affords extremely sensitive control yet which incorporates an inexpensive, readily available temperature responsive component.

The time delay relay 36 of the unit 30 is, likewise, a readily available commercial device. A thermal type time delay relay has been found to be suited for the unit, such relay having a heater coil 34 which is required to be energized for a preset time interval before the relay contacts are actuated. Such relays are commercially available as enclosed, plug-in type elements.

Referring also to FIG. 3, assuming for purposes of illustration a typical storage room for food products exhibiting a change in temperature under operating conditions of 4 in 30 minutes, with the usual bi-metallic or bulb-type thermostat responsive to such 4 temperature differential controlling the liquid solenoid valve V, the temperature curve for the storage room will have a sawtoothed wave form such as the curve A in FIG. 3. It is further assumed that with the refrigeration unit operating at its rated capacity, the temperature of the air in the room will be cooled at the same rate: 4 per 30 minutes.

A control unit 30 constructed in accordance with the invention applied for controlling the refrigerating system will maintain the temperature in the storage room within a differential of about 0.2 F. Turning again to the chart of FIG. 3, the small amplitude, high frequency sawtoothed curve B illustrates the temperature change in the storage room with such a control.

In the present case, as above stated, the thermostat 33 of the temperature control unit is responsive within a temperature differential of 01F. Set on 30 F., the thermostat contacts 38a, 3812 close or make on rise to the make temperature of 30.05 and open upon decrease in temperature to the break temperature of 29.95 F.

Referring to curve C, it will be noted that starting at the left side of the chart at 0 time and at 29.95, the temperature in the storage room air is rising at the rate of 4 per 30 minutes. At this rate of temperature change, forty-five seconds is required for the temperature to rise .1 F. to the make temperature of 30.05 responsive to which the thermostat contacts 38a, 38!) close. A time delay relay 36 is chosen having a time delay which will be of suflicient duration to limit short cycling of the refrigerating system by preventing the temperature control unit from responding to short duration transient temperature conditions, yet will not introduce a delay which will allow the temperature curve to rise unreasonably above the temperature at which the thermostat contacts close. In the present case, a time delay relay with a 20 second time delay has been chosen for illustration purposes. With this relay, the relay contacts close 20 seconds after the thermostat contacts close, which, at the rate of temperature change depicted in FIG. 3, will carry the storage room temperature to a point just under 30.1 F., at which point the relay contacts 40 close. These relay contacts, in series with the liquid solenoid valve V, cause the latter to be energized thereby admitting liquid refrigerant to the evaporator coil. With the fans 21-23 operating, it is assumed this Will provide substantially instantaneous cooling effect to reverse the temperature curve depicting the storage room temperature. This has been shown as an abrupt change in temperature for illustrative purposes. Curve C then demonstrates a decreasing temperature with the same slope as the rise in temperature, which de crease is maintained past the temperature of 29.95", the point at which the thermostat contacts 38a, 38b open. The time delay relay requires, in the usual case, about 5 seconds to operate its contacts and thus 5 seconds after the thermostat contacts 38a, 38b open the relay contacts 40 open to deenergize the liquid solenoid valve V and thereby interrupt the fiow of liquid refrigerant to the evaporator coil. At this point the temperature curve reverses again to depict rise in storage room temperature.

Turning again to the curve at the left side of the chart in FIG, 3. depicting rise in temperature, several transient variations from the uniform rise of the curve are shown, such changes or fluctuations in temperature being caused by the storage room doors being opened, or the like. These temperature fluctuations are of relatively short duration, on the order of a few seconds in each. case illustrated, and thus the time delay relay coil will not be energized for sufficient duration responsive to any such transient temperature changes to cause the relay contacts to be energized. The introduction of a time delay thus limits short cycling due to temporary, short duration temperature fluctuations. It will be understood that the time delay introduced by the relay is chosen in relation to the temperature curve for the system, so as to prevent short cycling and yet provide the accuracy of response desired. While the differential of temperature held by the temperature control unit will be enlarged due to the delay introduced by the time delay relay (in the present case the range is extended to 0.2 F. differential as compared with the 0.1 F. differential for the thermostat) yet high accuracy of control is achieved without excessive short cycling of the refrigerating system.

Thus, the present invention provides a combined temperature and humidity control for a refrigerating system and also provides an inexpensive yet highly sensitive temperature responsive device for use in this and other controls.

.The combined temperature and humidity control provides regulation within close limits for both temperature and humidity conditions by circulating a constant, pre determined .volume of air past the refrigerating system heat extractor, and regulating the heat extracted responsive to temperature. This control further provides for regulating relative humidity upon change in the latter from within the prescribed limits, by adjusting the volume of air circulated.

I claim as my invention:

1. For use in a storage compartment refrigerating system having a source of liquid refrigerant, an evaporator connected to said source, a liquid refrigerant valve, and power means for operating said valve to interrupt the flow of refrigerant from said source to said evaporator, which valve when opened admits refrigerant to said evaporator; a temperature control unit comprising, in combination, a temperature sensitive element for responding to the temperature in said storage compartment and having electrical circuit means which make and break responsive to compartment temperature rise and fall, respectively, to make and break temperatures within a specified temperature differential, and a time delay relay having an energizing element electrically connected to said electrical circuit means and having relay contacts for controlling the supply of power to said valve operating means.

2. In a refrigerating system for a storage compartment, the combination comprising, a source of liquid refrigerant, an evaporator connected to said source, a solenoid actuated liquid control valve in said connection operable to interrupt the flow of refrigerant, which valve when opened admits liquid refrigerant to said evaporator, power means for circulating compartment air over said evaporator, and means for controlling the temperature in said compartment Within upper and lower limits separated by a predetermined temperature differential including a temperature sensitive element for responding to the temperature in said storage compartment and having electrical circuit means which make and break contact in response to compartment temperature rise and fall, respectively, to make and break temperatures within a differential narrower than said predetermined differential, and a time delay relay operated by said electrical circuit means upon rise to make temperature and having circuit closing means operated upon said make temperature being maintained for the delay period of the relay to energize the solenoid of said liquid control valve, said delay period being such that compartment temperature remains under said upper limit.

3. In a refrigerating system for a storage compartment, the combination comprising, a source of liquid refrigerant, an evaporator connected to said source, a solenoid actuated liquid control valve in said connection operable to interrupt the flow of refrigerant, which valve when opened admits liquid refrigerant to said evaporator, power means for circulating compartment air over said evaporator, and means for controlling the temperature in said compartment within upper and lower limits separated by a narrow differential of about 0.2 F. including a temperature sensitive element for responding to the temperature in said storage compartment and having electrical circuit means which make and break contact in response to compartment temperature rise and fall, respectively, to make and break temperatures within a differential of about 0.1 F., and a time delay relay operated by said electrical circuit means upon rise to make temperature and having circuit closing means operated upon said make temperature being maintained for the delay period of the relay to energize the solenoid of said liquid control valve, said delay period being such that compartment temperature remains under said upper limit.

4. In a refrigerating system for maintaining given temperature and nearly saturated relative humidity conditions in a storage compartment, the combination comprising, a source of liquid refrigerant, an evaporator connected to said source, means for interrupting the flow of liquid refrigerant from said source to said evaporator, which interrupting means when opened admits liquid refrigerant to said evaporator, means including a temperature control unit operating in a temperature range on the order of plus or minus .l F. controlling said interrupting means to maintain storage compartment temperature within a narrow temperature differential, the lower limit of which is above the dew point for air at the given temperature and relative humidity conditions, means for circulating air between said storage compartment and evaporator in sufficient volume to maintain cooled air above the dew point so that any marginal condensation onsaid evaporator will be carried into said storage compartment, and means responsive to an increase in relative humidity in said storage compartment above the given relative humidity condition for reducing the volume of air circulated so that the temperature of cooled air is dropped below the dew point thereby to deposit moisture from said air in the form of condensation or frost on the evaporator coils.

5. In a refrigerating system for maintaining given temperature and nearly saturated relative humidity conditions in a storage compartment, the combination comprising, a source of liquid refrigerant, an evaporator connected to said source, means for interrupting the flow of liquid refrigerant from said source to said evaporator, which interrupting means when opened admits liquid refrigerant to said evaporator, means including a temperature control unit operating in a temperature range on the order of plus or minus .l F. controlling said interrupting means to maintain storage compartment temperature within a narrow temperature differential, the lower limit of which is above the dew point for air at the given temperature and relative humidity conditions, means for circulating air between said storage compartment and evaporator in sufficient volume to maintain cooled air above the dew point so that any marginal condensation on said evaporator will be carried into said storage compartment, humidistat means responsive to an increase in relative humidity in said storage compartment above the given relative humidity condition for reducing the volume of air circulated so that the temperature of cooled air is dropped below the dew point thereby to deposit moisture from said air in the form of frost on the evaporator coils, and said humidistat means also being responsive to a reduction in relative humidity in said storage compartment to about the specified condition to increase the volume of air circulated and thereby reduce moisture deposit from the circulated air.

6. A refrigerating system including means for maintaining a predetermined temperature and nearly saturated relative humidity in a refrigerated space, comprising in combination, a heat absorber, adjustable means for circulating air at a first rate between said absorber and said refrigerated space, means operating in a temperature range on the order of plus or minus .1 F. and responsive to temperature of said circulated air for adjusting the cooling effect of said heat absorber tending to maintain said air within a predetermined narrow temperature differential just above the dew point temperature for said predetermined relative humidity so that any marginal condensation on said heat absorber will be carried into said refrigerated space, and control means responsive to increase in relative humidity of said'circulated air above said predetermined relative humidity for adjusting said air circulating means to reduce the rate of circulation to a lower rate providing air temperature below said dew point temperature to condense moisture on said heat absorber and thereby decrease the relative humidity of said air, said control means being further responsive to decrease of relative humidity of said circulated air to below said predetermined relative humidity for adjusting said circulating'means to provide said first rate of circulation.

7. In a refrigerating system for maintaining given temperature and relative humidity conditions in a storage compartment, the combination comprising in combination, a source of liquid refrigerant, an evaporator connected to said source, means for interrupting the flow of liquid refrigerant from said source to said evaporator, which interrupting means when open admits liquid refrigerant to said evaporator, means including a temperature control unit controlling said interrupting means to maintain storage compartment temperature within a temperature differential, the lower limit of which is above the dew point for air at the given temperature and relative humidity conditions, means for circulating air between said storage compartment and the evaporator in sufficient volume to maintainthe cooled air above the dew point, means responsive to an increase in relative humidity in said storage compartment above the given relative humidity condition for reducing the volume of air circulated so that the temperature of cooled air is dropped below the dew point thereby to deposit moisture from said air in the form of condensation or frost on the evaporator coils, and a humidity control override unit comprising an additional temperature sensitive means for responding to the temperature in said storage compartment and having means for bypassing said relative humidity sensitive means in response to the storage compartment temperature rising above a predetermined range of temperatures.

8. A refrigerating system including means for maintaining predetermined temperature and relative humidity conditions in a refrigerated space, comprising in combination, a heat absorber, adjustable means for circulating air at a first rate between said absorber and said refrigerated space, means responsive to temperature of said circulated air for adjusting the cooling effect of said heat absorber tending to maintain said air within a predetermined narrow temperature ditferential just above the dew point temperature for said predetermined relative humidity so that any marginal condensation of said heat absorber will be carried into said refrigerated space, control means responsive to increase in relative humidity of said circulated air above predetermined relative humidity for adjusting said air circulating means to reduce the rate of circulation to a lower rate providing air temperature below said dew point temperature to condense moisture on said heat absorber and thereby decrease the relative humidity of said air, said control means being further responsive to a decrease in relative humidity of said circulated air to below said predetermined relative humidity for adjusting said circulating means to provide said first rate of circulation, and a humidity control. override unit comprising an additional temperature sensitive means for responding to the temperature in said refrigerated space and having means for bypassing said relative humidity responsive control means in response to the temperature in said refrigerated space rising above the predetermined range of temperatures.

9. A refrigerating system according to claim 8 in which said narrow temperature differential is about 02 F References Cited by the Examiner UNITED STATES PATENTS 1,793,857 2/1931 Kettering 62-158 1,955,192 4/1934 Kettering 62--176 2,203,560 6/1940 Ashley 62 -176 2,236,058 3/1941 Henney 62-180 2,296,530 9/1942 McGrath 62176 2,461,760 2/ 1949 Newton 62205 2,851,221 9/1958 Krogh 236-45 3,012,412 12/1961 Mufi'ly' 62176 FOREIGN PATENTS 624,443 7/1961 Canada.

MEYER PERLIN, Primary Examiner.

ROBERT A. OLEARY, Examiner. 

1. FOR USE IN A STORAGE COMPARTMENT REFRIGERATING SYSTEM HAVING A SOURCE OF LIQUID REFRIGERANT, AN EVAPORATOR CONNECTED TO SAID SOURCE, A LIQUID REFRIGERANT VALVE, AND POWER MEANS FOR OPERATING SAID VALVE TO INTERRUPT THE FLOW OF REFRIGERANT FROM SAID SOURCE TO SAID EVAPORATOR, WHICH VALVE WHEN OPENED ADMITS REFRIGERANT TO SAID EVAPORATOR; A TEMPERATURE CONTROL UNIT COMPRISING, IN COMBINATION, A TEMPERATURE SENSITIVE ELEMENT FOR RESPONDING TO THE TEMPERATURE IN SAID STORAGE COMPARTMENT AND HAVING ELECTRICAL CIRCUIT MEANS WHICH MAKE AND BREAK RESPONSIVE TO COMPARTMENT TEMPERATURE RISE AND FALL, RESPECTIVELY, TO MAKE AND BREAK TEMPERATURES WITHIN A SPECIFIED TEMPERATURE DIFFERENTIAL, AND A TIME DELAY RELAY HAVING AN ENERGIZING ELEMENT ELECTRICALLY CONNECTED TO SAID ELECTRICAL CIRCUIT MEANS AND HAVING RELAY CONTACTS FOR CONTROLLING THE SUPPLY OF POWER TO SAID VALVE OPERATING MEANS. 